The invention relates to a method of designing a cutting insert for a rotary milling tool or cutter useful in forming an external, generally-arcuate surface on a workpiece.
The prior art teaches myriad methods for forming an external, generally-arcuate surface of a part, for example, an outside diameter of a generally-cylindrical part. Under one prior art approach, an internal-diameter (ID) cutter is provided that includes a cutter body that is rotatable about a first axis. The cutter body includes an axial bore whose inner wall supports a plurality of identical tool inserts such that the cutting edges of each insert project generally radially-inwardly into the body""s axial bore. Prior to engagement of any of the inserts with the work, the cutter body is axially advanced relative to the work such that the first axis is positioned in parallel with the nominal or reference axis of the outer surface.
The cutter body is then rotated about the first axis and the cutter body is radially advanced relative to the reference axis, such that a like cutting edge on each insert serially engages the work to thereby generate the desired external contour. The axis of the rotating cutter body is thereafter orbited about the reference axis of the work in a plane generally orthogonal to the reference axis to thereby generate the rest of the desired external contour. In the event that the finished part includes a radial flange, the nominal inner diameter defined by the cutting edges of the inserts relative to the rotational axis of the tool body is made greater than the maximum diameter of the radial flange, and the rotational axis of the tool body is orbited around the nominal longitudinal axis of the cylindrical part to thereby simultaneously define both the radial flange and the nominal outside surface of the part.
Significantly, in order improve chip evacuation while further reducing tool pressure upon initial engagement with the work, the prior art teaches supporting the inserts in the cutter body such that each insert""s cutting edge is located in the circular cutting path of the tool and is inclined or canted with respect to the rotary axis by a predetermined helix angle (also known as an axial rake angle). Such canted inserts, in turn, require a helical cutting edge to ensure that the relief angle formed along the entire length of the cutting edge, between the cutting face and a series of reference lines intersecting the rotary axis and the cutting edge when viewed in cross-section, remains substantially constant. The shape of the helical cutting edge of such canted inserts is often empirically defined through trial and error as a series of three-dimensional coordinates. Such empirical definition is further complicated if the resulting tool will be used to simultaneously form a second, relatively-raised arcuate surface contour on the work immediately adjacent to a first convex surface portion of relatively-lesser nominal diameter, for example, as when defining an adjacent radial flange on a cylindrical protrusion of a workpiece. A typical canted-edge insert produced in accordance with the prior art, featuring an empirically-determined double-helix curved cutting edge, is illustrated in FIGS. 10 and 11.
Once defined, the helical cutting edge is typically formed on each insert with a CNC machine using a piecewise-linear function (wherein the CNC machine moves in a series of short, straight lines between the several empirically-defined three-dimensional coordinates), and often requiring a five-axis grinder. In addition to the cost associated with empirically-defining the cutting edges and otherwise producing such inserts, the resulting piecewise-linear cutting edges of such canted inserts often produce finished surfaces characterized by greater dimensional variation than their noncanted counterparts.
It is an object of the invention to provide a method for making a cutting insert for use in an ID rotary cutter, wherein the insert includes a cutting edge that is supported at a nonzero helix angle relative to the rotational axis of the cutter, that overcomes the deficiencies of the prior art.
It is another object of the invention to provide a method for making a cutting insert for use in an ID rotary cutter that does not require an empirical determination of a plurality of cutting edge coordinates with which to generate a desired external, convex contour of a milled part.
It is a further object of the invention to provide a cutting edge on an insert for use in an ID rotary cutter that lies within a common reference plane to thereby reduce the cost and complexity associated with manufacture of the insert.
Under the invention, a method is provided for making a cutting insert useful for milling, with an ID rotary cutter, an outer surface of a workpiece whose desired contour includes a first surface portion having a minimum radius relative to a reference axis, and a second portion having a maximum radius relative to the reference axis. An exemplary method includes selecting a first reference plane intersecting both the reference axis and the outer surface, and obtaining a first profile of the desired contour as the intersection of the outer surface with the first reference plane. The exemplary method also includes translating the first profile within the first reference plane in a direction normal to the reference axis such that the first profile is no less than a minimum distance away from the first axis, the minimum distance being greater than the maximum radius and, preferably, equal to the sum of the maximum radius plus a predetermined safety factor. The method also includes rotating the translated first profile about the reference axis to generate a reference contour that constitutes an expanded version of the original, desired profile. Significantly, however, because the generated reference contour is based upon the translated first profile, the difference between the reference contour""s minimum and maximum radii is identical to the difference between the original, desired contour""s minimum and maximum radii.
The exemplary method further includes selecting a second reference plane disposed at a predetermined helix angle relative to the reference axis and intersecting the reference contour, obtaining a second profile of the reference contour as the intersection of the reference contour with the second reference plane, and forming a cutting edge of the insert based at least in part on the second profile. The helix angle is preferably selected based at least in part upon the design of the finished part, for example, such that the second reference plane intersects each longitudinal end of the generated reference contour. As a result, the second reference plane will necessarily likewise intersect the outer surface of the finished part along its entire finished length of the workpiece to be formed upon engagement of the insert""s cutting edge. It will be appreciated that the material from which the part is to be milled is also accommodated when selecting the appropriate helix angle.
In accordance with a feature of the invention, the step of forming the insert""s cutting edge preferably includes programming a CNC machine to follow the second profile. While the CNC machine is preferably capable of directly receiving CAD-generated profile information with which to form the cutting edge, the invention alternatively contemplates that the second profile be approximated using a plurality of three-dimensional coordinates. It will be appreciated that, because the cutting edge lies within the second reference plane, cutting inserts in accordance with the invention can conveniently be manufactured using three- or four-axis grinders, rather than the five-axis grinders required by the typical prior art design.
In accordance with another feature of the invention, a method for making a cutting insert for an ID rotary cutter useful for milling a cylindrical part of nominal diameter is also advantageously provided. Specifically, a method for making such a cutting insert includes selecting a first reference plane intersecting both the reference axis and the outer surface, and obtaining a first profile of the desired contour of the outer surface as the intersection of the outer surface with the first reference plane. The method further includes translating the first profile within the first reference plane in a direction normal to the reference axis such that the first profile is no less than a minimum distance away from the reference axis, the minimum distance being greater than the nominal radius; rotating the translated first profile about the reference axis to generate a reference contour; and projecting the reference contour onto a second reference plane that is disposed at a predetermined helix angle relative to the first reference plane.
In accordance with yet another feature of the invention, a cutting tool employing these cutting inserts beneficially provides a consistent tool pressure and cutting rate because the cutting edge of each insert forms a straight cutting edge when viewed from within the second reference plane. Additional benefits may include improved tool life and reduced tool chatter.